Wow, double clicks... never seen that before. What's the purpose of it?

If you middle-click a square, all non-flagged tiles around it are revealed.

So, if you see a "#" tile and you've flagged # bombs adjacent to it, you can safely middle-click that tile and have all other adjacent unknown tiles revealed. It's a time-saver. Of course, it can bite you in the butt if you click the wrong tile or mis-flag something.